In operation of a screw or worm extruder with a plastic mass, unfortunately, from time to time foreign bodies, particularly metal pieces, are fed together with the mixture to be processed into the extruder.
These foreign bodies according to their shape and size cause deformations and fractures along the extrusion path. The damaged portions of the extrusion path usually cause heavy subsequent damage which frequently makes impossible further manufacturing with the extruder although the additional damage may not appear to be significant. That is particularly true for an extruder with a pin cylinder which is provided with an assembly of pins projecting into the extrusion chamber interrupting the extrusion path and which may be broken away.
This so-called pin extruder has had an increasing acceptance in the extruder market in recent years because of the higher output and lower extrusion temperature of this extruder in comparison with conventional extruder units.
An extruder with individual pin monitoring is taught in German Pat. No. 32 21 472. The disadvantage of the approach described in that patent document is the need to have the large number of pins each provided with means for signaling a pin break in order to carefully monitor the unit. When one considers that a pin extruder has more than 100 pins it is clear that the cost for such a pin break detector must be considerable. Also the plurality of connections between the pins and the drive shut off device are susceptible to operational disturbance by damage from the outside.